JP2009248000A - Manufacturing method of electret filter medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for effectively charging a nonwoven fabric having the fiber diameter of the nano order. <P>SOLUTION: By impregnating the nonwoven fabric having the fiber diameter of the nano order with liquid and deliquoring it, a charging performance is improved and stabilized without damaging a nanofiber layer. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for producing a filter medium obtained by electretizing a nonwoven fabric composed of fibers having an average fiber diameter of 0.05 to 1 μm.

    In recent years, nano-order fibers (nanofibers) having a fiber diameter of 1 μm or less have been widely studied. Examples of a method for producing an aggregate of fibers having a small fiber diameter include a composite spinning method, a high-speed spinning method, and a charged spinning method. Among them, the charged spinning method can be spun easily and with fewer man-hours than other methods. A high voltage is applied to a liquid (for example, a solution containing a polymer that forms a fiber, or a molten polymer) to charge the liquid, causing the liquid to move toward a counter electrode material, and forming a fiber. Usually, the polymer forming the fiber is spun out of the solution and forms the fiber until it is captured by the counter electrode material. For example, when a solution containing a polymer that forms a fiber is used, the fiber is formed by solvent evaporation, when a molten polymer is used, by cooling, or by chemical curing (treatment with a curing vapor). ). Further, the obtained fiber is collected on a collecting body arranged as necessary, and if necessary, peeled from the collecting body and can be used as a fiber aggregate. Further, since it is possible to directly obtain an aggregate of non-woven fibers, there is no need to form an aggregate of fibers after spinning the fibers once as in other methods, and the operation is simple. In addition, the nonwoven fabric is electretized by the charged spinning itself, but the charged state is unstable and the charge amount is not sufficient (for example, see Patent Documents 1 to 3).

  The collection efficiency can be significantly improved by electretizing the nonwoven fabric. There are electretization methods such as electretization by corona discharge and electretization by water current collision with sufficient pressure, but nanofibers are extremely fine fibers, so the rigidity of the fibers is small, and such electretization is possible. In the treatment, fuzz of the nanofiber part occurs after the electret formation, breakage, etc., and not only high collection performance cannot be expected as a filter medium, but partial particle leakage occurs, and it is difficult to ensure stable quality (For example, refer to Patent Documents 4 and 5).

  As an electretization method capable of relatively suppressing damage to the nonwoven fabric, a method of quickly drying after passing a liquid mixture with an organic solvent or water through the nonwoven fabric (see, for example, Patent Documents 6 and 7). These may be squeezed with a nip roll during liquid impregnation, or the liquid is sufficiently immersed by passing through a process such as sucking a sheet soaked with a slit-like nozzle, or the liquid is removed from the nonwoven fabric. By adding such an operation, damage to the nanofiber cannot be fully avoided, and it is difficult to ensure stable quality.

Japanese Patent Publication No. 48-1466 JP-A 63-145465 JP 2002-249966 A JP 2008-682 A Japanese National Patent Publication No. 9-501604 Special table 2003-514998 gazette JP 2003-73971 A

  The present invention has been made to solve the above-described problems, and provides an electretization method that effectively improves the collection efficiency of a filter medium using nanofibers.

The present invention is as follows.
1. An electret filter medium comprising electrifying a non-woven fabric having an average fiber diameter of 0.05 to 1 μm by impregnating and draining the non-woven fabric with a liquid that is at least twice as high as the dielectric constant of the non-woven fabric and draining the non-woven fabric. Manufacturing method.
The non-woven fabric described in 2.1 includes a fiber obtained by producing a solution containing a polymer and an organic solvent as main components and then spinning the solution using a charged spinning method. A method for producing an electret filter medium.
3.1 A method for producing an electret filter medium, wherein the nonwoven fabric according to 1 or 2 and a nonwoven fabric having a fiber diameter larger than the fiber diameter of the fibers constituting the nonwoven fabric are laminated.

  The present invention provides an effective electretization method for a nonwoven fabric having a fiber diameter of nano-order fibers, which can obtain a high-quality electret without causing damage to the fabric.

  The polymer used in the nonwoven fabric of the present invention is preferably one that retains high electrostatic permanent chargeability by charge treatment. For example, polyethylene, polypropylene, polystyrene, poly-4 methyl-1 pentene, poly-3 methyl-1 butene, polyphenyl sulfide, polyether I-terketone, polyether sulfone, polyarylate, polyamideimide, polyetherimide, polyimide , Polybenzoxazole, polysulfone, polyarylate, polybutylene terephthalate, polycarbonate, wholly aromatic polyester, polytetrafluoroethylene, chlorotrifluoroethylene-ethylene copolymer, polyvinylidene fluoride, polyvinyl fluoride, polychlorotrifluoroethylene Tetrafluoroethylene-ethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-perfluoro It can be used an alkyl vinyl ether copolymer or a mixture thereof.

  In the nonwoven fabric of this invention, additives, such as an inorganic or organic filler, can also be mix | blended as what improves the various characteristics of the nonwoven fabric obtained by spinning.

  It is preferable that the nonwoven fabric of this invention is formed from the fiber whose average fiber diameter is 0.05-1 micrometer. If the average fiber diameter is smaller than 0.05 μm, the self-supporting property is poor, which is not preferable. On the other hand, if the average fiber diameter is larger than 1 μm, the surface area becomes small and the effect of electret is reduced, which is not preferable. A more preferable average fiber diameter is 0.06 to 0.8 μm.

  The method for producing the nonwoven fabric of the present invention is not particularly limited as long as it is a method for obtaining a nonwoven fabric having an average fiber diameter of 0.05 to 1 μm, but a charged spinning method is preferred. Hereinafter, a method for producing by the charge spinning method will be described in detail.

  The charged spinning method used in the present invention is a kind of solution spinning. Generally, a positive high voltage is applied to a polymer solution, and fiber formation is performed in the process of being sprayed on a grounded or negatively charged surface. It is a technique to wake you up. An example of the charge spinning apparatus is shown in FIG. In the figure, a charged spinning device 1 is provided with a spinning nozzle 2 that discharges a polymer that is a raw material of a fiber, and a counter electrode 5 that faces the spinning nozzle 2. The counter electrode 5 is grounded. The polymer solution charged with a high voltage jumps out from the spinning nozzle 2 toward the counter electrode 5. At that time, it is fiberized. A solution in which a polymer is dissolved in an organic solvent is discharged into an electrostatic field formed between electrodes, the solution is spun toward a counter electrode, and the fibrous material that is formed is accumulated on a collection substrate to form a nonwoven fabric. Obtainable. The term “nonwoven fabric” as used herein refers not only to the state in which the solvent of the solution has already been distilled off to form a nonwoven fabric, but also to the state in which the solvent of the solution is included.

  In the case of a nonwoven fabric containing a solvent, the solvent is removed after the charged spinning. Examples of the method for removing the solvent include a method of immersing in a poor solvent and extracting the solvent, a method of evaporating the residual solvent by heat treatment, and the like.

  The material of the solution tank 3 is not particularly limited as long as it is resistant to the organic solvent used. Further, the solution in the solution tank 3 can be discharged into the electric field by a mechanical push-out system, a pump-out system, or the like.

  The spinning nozzle 2 preferably has an inner diameter of about 0.1 to 3 mm. The nozzle material may be made of metal or non-metal. If the nozzle is made of metal, the nozzle can be used as one electrode. If the nozzle 2 is made of non-metal, an electric field is applied to the extruded solution by installing an electrode inside the nozzle. Can be made. In consideration of production efficiency, it is possible to use a plurality of nozzles. In general, a nozzle having a circular cross section is used, but a nozzle having a different cross section may be used depending on the type of polymer and intended use.

  As the counter electrode 5, various shapes of electrodes can be used depending on the application, such as a roll-shaped electrode, a flat plate-shaped, or a belt-shaped metal electrode shown in FIG. 1.

  Moreover, although description so far is a case where an electrode serves as the board | substrate which collects a fiber, you may collect a fiber there by installing the thing used as the board | substrate collected between electrodes. In this case, for example, continuous production is possible by installing a belt-like substrate between the electrodes.

  Moreover, although it is common to form with a pair of electrodes, it is also possible to introduce a different electrode. It is also possible to perform spinning with a pair of electrodes, and to control the spinning state by controlling the electric field state with electrodes having different potentials introduced.

  Although the voltage application apparatus 4 is not specifically limited, A direct current high voltage generator can be used and a Van de Graf electromotive machine can also be used. The applied voltage is not particularly limited, but is generally 3 to 100 kV, preferably 5 to 50 kV, and more preferably 5 to 30 kV. Note that the polarity of the applied voltage may be either positive or negative.

  The distance between the electrodes depends on the charge amount, the nozzle size, the spinning solution flow rate, the spinning solution concentration, and the like, but a distance of 5 to 20 cm was appropriate when it was 5 to 120 kV.

  Examples of the solvent of the polymer include acetone, chloroform, ethanol, isopropanol, methanol, toluene, tetrahydrofuran, water, benzene, benzyl alcohol, 1,4-dioxane, propanol, carbon tetrachloride, cyclohexane, cyclohexanone, methylene chloride, phenol, Highly volatile solvents such as pyridine, trichloroethane, acetic acid, N, N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N, N-dimethylacetamide (DMAc), 1-methyl-2-pyrrolidone (NMP) , Ethylene carbonate, propylene carbonate, dimethyl carbonate, acetonitrile, N-methylmorpholine-N-oxide, butylene carbonate, γ-butyrolactone, diethyl carbonate, diethyl Ether, 1,2-dimethoxyethane, 1,3-dimethyl-2-imidazolidinone, 1,3-dioxolane, ethyl methyl carbonate, methyl formate, 3-methyl oxazolidine-2-one, methyl propionate, 2 -A solvent having relatively low volatility such as methyltetrahydrofuran and sulfolane. Alternatively, it is possible to use a mixture of two or more of the above solvents.

  The spinning atmosphere is generally performed in the air, but by performing charge spinning in a gas having a higher discharge starting voltage than air such as carbon dioxide, spinning at a low voltage is possible, such as corona discharge. It is also possible to prevent abnormal discharge. Further, when water is a poor solvent for the polymer, polymer precipitation may occur near the spinning nozzle. Therefore, in order to reduce the moisture in the air, it is preferable to carry out in the air that has passed through the drying unit.

  Next, the step of obtaining the nonwoven fabric accumulated on the collection substrate will be described. In the present invention, while spinning the solution toward the collection substrate, the solvent evaporates depending on conditions to form a fibrous material. At normal room temperature, the solvent completely evaporates until it is collected on the collection substrate. However, if the solvent evaporation is insufficient, the solvent may be drawn under reduced pressure. The fibers of the present invention are formed at the latest when collected on the collection substrate. Further, the temperature at which the spinning is performed depends on the evaporation behavior of the solvent and the viscosity of the spinning solution, but is usually 0 to 50 ° C. And a porous fiber is further accumulated on a collection board, and a nonwoven fabric is manufactured.

  The basis weight of the nonwoven fabric of the present invention is determined according to the intended use and is not particularly limited, but is preferably 0.05 to 50 g / m2. The basis weight here is in accordance with JIS-L1085. If it is 0.05 g / m 2 or less, the filter collection efficiency is unfavorably low, and if it is 50 g / m 2 or more, the filter ventilation resistance becomes too high, which is not preferable.

  The thickness of the nonwoven fabric of the present invention is determined according to the intended use and is not particularly limited, but is preferably 1 to 100 μm. The thickness here is measured with a micrometer.

  If necessary, the nonwoven fabric of the present invention can be post-treated so as to be adapted to various uses. For example, a calendar process, a hydrophilic process, a water repellent process, a surfactant adhesion process, a pure water washing process, etc. for densification or adjusting the thickness accuracy can be performed.

  Although the nonwoven fabric obtained by this invention may be used independently, according to a handleability and a use, you may use it in combination with another member. For example, a cloth (nonwoven fabric, woven fabric, knitted fabric) that can serve as a support substrate as a collection substrate, a film, a drum, a net, a flat plate, a belt, a conductive material made of metal or carbon, a non-polymer made of an organic polymer, etc. Conductive materials can be used. By forming a non-woven fabric thereon, a member in which the support base material and the non-woven fabric are combined can be created.

  As the fabric that can serve as the support substrate, a nonwoven fabric is most preferably used from an economical viewpoint. The fiber diameter constituting the nonwoven fabric of the support substrate is desirably larger than the fiber diameter of the charged nonwoven fabric of the present invention. The nonwoven fabric of the supporting substrate is effective for increasing the rigidity as a filter and preventing the deformation of the filter. For the above purpose, the fiber diameter constituting the nonwoven fabric of the supporting substrate is preferably 1.5 times or more, more preferably 2 times or more, particularly preferably the fiber diameter of the nonwoven fabric of the present invention subjected to the charge treatment. The fiber diameter is 5 times or more. When the fiber diameter is 500 times or more, it may be difficult to join the two nonwoven fabrics.

  The nonwoven fabric obtained by this invention can improve filter performance markedly by carrying out the electretization process. Since the nanofiber is a very fine fiber, the electretization method needs to be an electret method that has low fiber rigidity and suppresses damage to the fiber as much as possible. Specifically, a method of impregnating and draining a liquid can be mentioned. A liquid is applied to the above-described nonwoven fabric to obtain a sufficiently infiltrated state, and the infiltrated nonwoven fabric is dried. The liquid needs to be in a state where the nonwoven fabric is evenly and sufficiently permeated, so that the liquid on the opposite side is permeated to the suction side by giving suction to one side of the nonwoven fabric after or while applying the liquid to the nonwoven fabric. This is very important.

  Since the liquid penetrates through the non-woven fabric by the suction action, and evenly penetrates the entire non-woven fabric and then drains, it can be made uniform and high-quality electret non-woven fabric.

  The method for applying the liquid to the nonwoven fabric may be a method of immersing in a water tank as long as it can be immersed in the liquid or on the liquid surface.

  The suction of the liquid is preferably performed by placing the nonwoven fabric on a mesh-like filter so as to suppress damage to the nanofibers. The mesh size of the surface in contact with the nonwoven fabric is preferably 500 μm or less in terms of the maximum pore diameter. More preferably, it is 300 μm or less. The shape and material of the mesh are not particularly limited, and examples thereof include a mesh using a glass material, a ceramic material, etc., or a plain weave, a weave, a tatami, an expanded metal, a spark mesh, a crimp, and punching.

  Further, after impregnating and draining the liquid, drying strengthening may be further performed by a conventionally known hot air drying method, vacuum drying method, natural drying method or the like.

In the present invention, the liquid used for electretization can be water, an organic solvent or the like alone or in a mixture. In addition, a liquid having a high dielectric constant, specifically, a liquid having a dielectric constant that is at least twice that of the fibers constituting the electret nonwoven fabric is preferred for obtaining good charging performance. When water is used for the mixed liquid, it is preferable to use water as clean as possible. In particular, pure water such as ion-exchanged water, distilled water, filtered water permeated through a reverse osmosis membrane is preferable. The level of pure water is preferably 10 ³ μS / m or less, more preferably 10 ² μS / m or less in terms of conductivity.

  Particularly, it is preferable to mix a water-soluble organic solvent with water so that the permeability to the nonwoven fabric can be further improved, and the nonwoven fabric can be more smoothly impregnated and drained.

  The type of the water-soluble organic solvent is not particularly limited as long as the mixed solution has good permeability to the nonwoven fabric. For example, alcohols such as methyl alcohol, ethyl alcohol and isopropyl alcohol, ketones such as acetone and methyl ethyl ketone, esters such as propyl acetate and butyl acetate, other aldehydes and carboxylic acids can be used. In particular, alcohols or ketones are preferable from the viewpoint of permeability, and it is particularly preferable to use at least one of acetone, isopropyl alcohol, and ethanol. More preferably, the main component is isopropyl alcohol.

  The use of the nonwoven fabric obtained by this invention can be used for various air filter uses, such as a filter for air cleaners, a filter for precision instruments, a cabin filter for automobiles, trains, etc., an engine filter, and a filter for building air conditioning. It is particularly suitable for air purification applications that require heat resistance, mechanical strength, and thermal dimensional stability.

  EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples. The evaluation items in the following examples were carried out by the following methods.

[Logarithmic viscosity]
A solution obtained by dissolving 0.5 g of polyamideimide resin in 100 ml of N-methyl-2-pyrrolidone was kept at 30 ° C. and measured using an Ubbelohde viscosity tube.

[Average fiber diameter]
A scanning electron micrograph (magnification of 10,000 times) of the surface of the obtained nonwoven fabric was taken, and an average value obtained by measuring the fiber diameter at n = 30 was calculated from the photograph.

[Glass-transition temperature]
The inflection point of the loss elastic modulus of dynamic viscoelasticity measured by applying a vibration of a frequency of 110 Hz to a polyamideimide film having a measurement width of 4 mm and a length of 15 mm using a DVE-V4 Rheospectr manufactured by Rheology, is the glass transition temperature. did.

[Evaluation of filter performance]
The ventilation resistance was determined by reading the difference in static pressure between the upstream and downstream of the filter with a pressure gauge by controlling the nonwoven fabric sample in a duct and controlling the linear velocity of the filter medium to 10 cm / sec. The particle collection efficiency was evaluated by measuring the concentration of particles having a particle diameter of 0.3 to 0.5 μm when air dust was passed through the filter medium at 10 cm / sec, using a particle counter (KC01-C manufactured by Rion Co., Ltd.). The upstream and downstream were measured, the transmittance was determined from the ratio of the upstream and downstream particle concentrations, and the QF value was further determined by the following formula. If the QF value improves, the filter performance is regarded as high performance.
QF (mmAq ⁻¹ ) = − ln (particle transmittance) / airflow resistance mmAq

Example 1
A four-necked flask equipped with a thermometer, a cooling tube, and a nitrogen gas inlet tube contains 1 mol of trimellitic anhydride (TMA), 0.995 mol of diphenylmethane diisocyanate (MDI), and 0.01 mol of potassium fluoride in a solid content concentration. Was prepared together with N, N-dimethylacetamide so as to be 25%, heated to 90 ° C. and stirred for about 3 hours to synthesize polyamideimide. The obtained polyamideimide had a logarithmic viscosity of 0.86 dl / g and a glass transition temperature of 290 ° C. N, N-dimethylacetamide was added to the obtained polyamideimide solution to make the solid content concentration 20%. Using the apparatus shown in FIG. 1, this polyamideimide solution was applied to a fibrous material collecting electrode 5 with a polyester nonwoven fabric having a basis weight of 80 g / m 2, a fiber diameter of 45 μm, and a thickness of 0.2 mm, and then discharged. An 18 G (inner diameter: 0.8 mm) needle was used for the spinning nozzle 2, the voltage was 15 kV, and the distance from the spinning nozzle 2 to the counter electrode 5 for collecting fibers was 10 cm. The obtained nonwoven fabric had an average fiber diameter of 0.25 μm, a basis weight of 0.5 g / m 2, and a dielectric constant of 4.0 (referred to as nonwoven fabric A). The obtained non-woven fabric is placed in close contact with the filter part of a Buchner glass filter (trade name Fine 25G3, maximum pore diameter of the filter part 30 μm), the filter is placed on a suction bottle, and isopropyl alcohol (dielectric constant) 18.3) and ion-exchanged water (dielectric constant 80.0) were mixed thoroughly in a volume ratio of 3: 7, and 100 cc of liquid (liquid level height of about 5 cm) was added, and the liquid was impregnated while sucking under reduced pressure. The liquid was drained. Then, the state investigation of the nanofiber part was observed, and there was no abnormality such as breakage, so the filter performance was evaluated. As a result, the QF before liquid impregnation was improved to 0.15 with respect to 0.09.

(Comparative Example 1)
The nonwoven fabric A was placed on an aluminum flat plate ground electrode, and a corona charging treatment was performed at a voltage of 20 kV for 10 seconds using a needle electrode from a distance of 1 cm from the nonwoven fabric. Not only was the fuzz of the nanofiber part generated, but it was also partially broken.

  It realizes and provides a filter medium having high filter performance by charging a non-woven fabric having an average fiber diameter of 0.05 to 1 μm, and has a large industrial and environmental value.

Schematic cross-sectional view of a charged spinning device

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Charge spinning apparatus 2 Spinning nozzle 3 Solution tank 4 High voltage power supply 5 Counter electrode (collection board)

Claims (3)

  In electretization of a nonwoven fabric having an average fiber diameter of 0.05 to 1 μm, an electret filter medium is characterized by impregnating and draining the nonwoven fabric with a liquid that is at least twice as high as the dielectric constant of the nonwoven fabric and electretizing Production method.   The nonwoven fabric according to claim 1 contains a fiber obtained by producing a solution mainly composed of a polymer and an organic solvent, and then spinning the solution by a charge spinning method using the solution. A method for producing an electret filter medium. A method for producing an electret filter medium, wherein the nonwoven fabric according to claim 1 or 2 is laminated with a nonwoven fabric having a fiber diameter larger than that of the fibers constituting the nonwoven fabric.

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